Channel Switch Announcements in CUWN

In this post we'll have a look at how the Channel Switch Announcement (CSA) IE could be present in beacons and actions frames from radios in a CUWN platform. In most of the literature (including CWAP from CWNP) and CUWN-stuff at CCO you get the impression that CSA, either as an IE in a beacon frame or an action frame is only relevant in a 802.11h radar detection and avoidance context but is actually also implemented when the system does dynamic channel assignments (DCA) if configured. And not only for the DFS-channels we have in UNII-2 and UNII-2e, we can also notice the CSA IE in UNII-1 during DCA events.

To test this I have a WLC 2504 running code 7.6.130 and some APs like 1142, 2602 and 3702 in the lab. To provoke a channel change I've set two radios statically to the same channel (ch40) and before RRM and DCA decides and adjusts new channels I set one of the radios to "Global" which in the CUWN-language means that the WLC is in charge of the channels via DCA and automatically should choose the best channel. A packet capture is started on the relevant channel to collect the frames and stopped after a change of channels can be verified in the WLC GUI.

To "enable" Channel Switch Announcements we must also enable it in the WLC since it is disabled by default and an optional feature according to the 802.11h-2003 amendment.

So after collecting the frames during a channel change due to DCA we can open it in a packet analyzer and have a look at the frames on a high level when one of the AP radios (10:BD:18:31:DF:DC) leave channel 40.

Here we can see that AP DF:DC stops the transmission of beacons but we can also notice an action frame. This action frame contain the CSA IE that informs the STA's (the ones that understand 802.11h) that the new channel will be 64 and this change is about to happen any time after this frame.

In the last five beacon frames from AP DF:DC we can also notice the same CSA IE:

Alright - so what is the take on this?

I guess it is generally nice to inform the STAs in the cell that there is a change of operating channel coming up, if the STAs then are capable of interpreting this IE and also take some kind of action like scanning only on the announced channel to reduce roaming time is another question that is more kind of open.

Enabling Channel Switch Announcements is not included in Cisco's "WLC Configuration Best Practices" found here, perhaps it should be? However in Willam Jones excellent guide "Tuning Cisco WLC for High Density Deployments" found here he suggest it is a good idea to enable CSA to inform the clients about an upcoming change.

/Fredrik